The present invention relates to a vacuum ejector that includes a nozzle which can be connected to a compressed air source for directing a jet of compressed air onto a circular plate located close to the nozzle orifice and essentially perpendicular to the center axis of said nozzle, said jet of air being deflected upon contact with the plate and expands radially outwards.
In a vacuum ejector of this kind the subpressure is taken out through an annular slot in the circular plate with its center in the extension of the center axis of the nozzle. The slot is connected to a vacuum chamber which can be connected to one or more suction cups for instance, by one or more conduits. When several suction cups are connected, there occurs the well known problem of balancing the flows in the conduits leading to the cups so as to avoid concentrating the major part of the flow on one or more suction cups that have not engaged a generally air-impermeable surface in the process of a lifting operation for instance. In such cases the remaining suction cups lose their ability to fasten to the surface, either totally or partially. This problem is normally alleviated by providing each suction cup with a valve which is normally closed. A valve operating means includes a pin which projects out in front of the plane of the suction cup and opens the valve when the suction cup is pressed against a surface. Alternatively, each suction conduit may be provided with a valve of the kind that is closed when the flow exceeds a predetermined limit. Both types of valves are expensive and one valve or a pair of valves is required for each suction cup, which adds greatly to the cost of an ejector device that includes many suction cups. Another drawback is that no standard valves are available for small suction cups, e.g. cups having a diameter of 10-15 mm. Each ejector will often include from three to ten suction cups of this small size.
The object of the present invention is to provide a vacuum ejector that includes a plurality of suction devices with which the air flows are balanced-out in a simple and effective manner. This object is achieved in accordance with the invention by providing a plurality of apertures in the circular plate at a determined radial distance from the center of the circular plate centrally opposite the point of impact of the compressed-air jet, and connecting said apertures with separate ducts each of which is connected to its respective suction device.
Trials have shown that when using an inventive vacuum ejector equipped, e.g., with six suction cups each operating with 50 NI/min, the ejector will only lose about 3% units of vacuum level when both mutually adjacent suction cup ducts are exposed to atmosphere and about 0.75-1.0% unit of vacuum level when one or more non-adjacent suction cup ducts are exposed to atmosphere.
The best result is achieved when the apertures in the plate are in the form of arcuate slots that have the same radius of curvature as the radial distance to the center of the plate. According to one preferred embodiment, the slot interspaces are a fraction of the length of the slots. In order to reduce the pressure drop in the ducts leading to the suction devices, the areas of said ducts are preferably substantially greater than associated apertures in the circular plate.